1. Field of the Invention
This invention relates in general to spin valve transistors for reading information signals from a magnetic medium and, in particular, to a spin valve transistor sensor using a magnetic tunnel junction device for improving magnetoresistive coefficient, and to magnetic storage systems which incorporate such sensors.
2. Description of the Related Art Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier layer 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
Differential GMR and MTJ sensors comprising dual SV or MTJ sensors, respectively, can provide increased magnetoresistive response to a signal field due to the additive response of the dual sensors connected in a differential circuit. However, even greater increases in magnetoresistive response may be obtainable from yet another type of GMR sensor known as a spin valve transistor (SVT) sensor.
A spin valve transistor sensor having a common base configuration and comprising an emitter Schottky barrier, a collector Schottky barrier and a ferromagnetic multilayer sandwiched between the silicon substrates of the two semiconductor elements was described by Monsma et al., Science, Vol. 281, 1998, pp. 407. Monsma et al. formed the SVT sensor by sandwiching a Co/Cu/Co/Pt multilayer between two semiconductor quality silicon (Si) substrates by a vacuum bonding technique. An emitter Schottky barrier was formed by a first Si semiconductor and the Pt metal layer and a collector Schottky barrier was formed by a second Si semiconductor and the outside Co layer of the Co/Cu/Co/Pt multilayer. The emitter Schottky barrier was negatively (forward) biased with a dc current source, and the collector Schottky barrier was positively (reverse) biased. The emitter bias accelerates electrons over the emitter Schottky barrier where they become hot electrons in the Co/Cu/Co/Pt multilayer common base. The number of hot electrons crossing the base and collected at the collector Schottky barrier is spin dependent due to the GMR effect in the multilayer common base. Magnetic alignment of the base layers by an external magnetic field results in increased collector current. The SVT sensor is expected to provide high magnetoresistance signals with high signal-to-noise ratios.
However, applications of the SVT sensor are hampered by a number of materials and process incompatibilities. First, the need for semiconductor quality silicon Schottky barriers requires very high temperature processing which is incompatible with formation of sharply defined layers of GMR-type sandwiched materials. Second, the vacuum bonding technique used to make SVT sensors is not suitable for mass fabrication of magnetic sensors and results in sensor thicknesses much greater than required for high density magnetic recording applications.
Therefore, there is a need for an SVT sensor that provides the advantages of improved magnetoresistive coefficient and high signal-to-noise ratio without the fabrication problems associated with the materials/process incompatibilities inherent in the use of semiconductor materials together with spin valve sandwich materials.
It is the object of the present invention to disclose a spin valve transistor (SVT) sensor using a magnetic tunnel junction (MTJ) element.
It is another object of the present invention to disclose an SVT sensor using an MTJ element having dimensions compatible with the requirements for magnetic recording sensors used in high density storage applications.
It is a further object of the present invention to disclose an SVT sensor having an MTJ element with high magnetoresistance coefficient and improved signal-to-noise ratio.
It yet another object of the present invention to disclose a process for making an SVT sensor having an MTJ element for use in high density storage applications.
In accordance with the principles of the present invention, there is disclosed a spin valve transistor (SVT) sensor having an emitter element, a collector element and a common base element disposed between the emitter and collector elements. The emitter element comprises an antiferromagnetic layer, a ferromagnetic pinned layer and an electrically insulating first tunnel barrier layer. The collector element comprises an electrically insulating second tunnel barrier layer and a nonmagnetic metal layer. A ferromagnetic free layer disposed between the emitter and collector elements provides the common base element of the SVT sensor. The antiferromagnetic layer, the ferromagnetic pinned layer, the insulating first tunnel barrier layer and the ferromagnetic free layer form a magnetic tunnel junction (MTJ) element.
The emitter element is biased negatively with respect to the base element and the collector element is biased positively with respect to the base element. The negative emitter allows an emitter-base electron current to flow by tunneling through the first tunnel barrier layer. A fraction of this emitter-base current is thermally excited (hot) electrons which can cross the base element layer and pass over the energy barrier of the second tunnel barrier layer which was lowered by the negative bias applied to the emitter element. This hot electron current is strongly spin polarized and, due to the GMR effect in the MTJ element, the magnitude of the current flowing into the collector element (base-collector current) is strongly dependent on the relative orientation of the magnetizations of the pinned and free ferromagnetic layers of the MTJ element. The fraction of the emitter-base current that is not sufficiently energetic to pass over the energy barrier of the second tunnel barrier layer or that is scattered while traversing the base element or scattered at the interfaces flows back to the emitter current supply via a common base electrode connected to the base element. Changes of the magnitude of the base-collector current are detected by a signal detector in the base-collector circuit and provide a sensitive measure of external magnetic (signal) fields from the surface of a magnetic recording disk or any other suitable signal source.
The SVT sensor of the present invention comprises thin layers of materials that may be vacuum deposited using methods known to the art that are compatible with the requirements for mass fabrication of magnetic sensors for high data density applications.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.